Optical recording has many desirable features such as high density, large capacity and non-contact with the record/reproduce head. To take advantage of these features, active efforts are being made toward development and commercialization of this new technology. Two types of media, magneto-optical recording medium and phase-change recording medium, have been the subject of particular interest because of the facility they offer in performing erasure and subsequent recording.
A magnetic material is used in the recording layer of magneto-optical recording media and information is recorded as a change in magnetization while the recorded information is read as a change in the state of polarization of reproducing light. Amorphous rare earth/transition metal alloy systems based on alloys of rare earth metals such as Gb, Tb and Dy and transition metals such as Fe, Co and Ni (e.g., GdCo, GdFe, TbFe, DyFe, GdTbFe, GdFeCo, TbFeCo and TbFeNi) are commonly used as magnetic materials since they exhibit superior magneto-optical effects. Amorphous rare earth/transition metal alloy systems are formed into thin recording layers by sputtering or other thin-film processes and incorporated in magneto-optical recording media.
A problem with amorphous rare earth/transition metal alloy systems is that they are highly susceptible to oxidation and that their magnetic characteristics, such as coercive force, will decrease upon oxidation. This causes time-dependent deterioration in the characteristics of magneto-optical recording media, thus making it impossible to record and reproduce information in a consistent manner. Therefore, preventing oxidation of the recording layer made of amorphous rare earth/transition metal alloy systems has been one of the major requirements to be met in order to commercialize magneto-optical recording media by improving the long-term reliability of their performance.
The other type of optical recording media, i.e., phase-change recording medium, commonly relies upon the mechanism of a crystalline to amorphous phase change, in which information is recorded in a heating mode and reproduced on the basis of the change in reflectance that occurs in response to the phase change. The recording layer in a phase-change recording medium include Te based alloy or non-Te based alloy.
Phase-change recording media have also suffered from the problem that the recording layer is affected by aerial oxygen or moisture to become deteriorated in its characteristics although this problem is not as serious as in the case of magneto-optical recording media. A need has therefore existed for improving the resistance of the characteristics of phase-change recording media against time-dependent deterioration before they are commercialized.
Various methods have been proposed to prevent time-dependent deterioration of the recording layer in magneto-optical recording media. According to one proposal, metals such as Al, Ti, Cr and Pt are incorporated in the recording layer but it has been impossible to attain marked improvement in corrosion resistance and weathering properties.
It has also been proposed that protective layers be used to prevent time-dependent deterioration of the recording layer and this approach is roughly divided into two types, one employing protective layers made of inorganic materials, and the other using organic materials.
Known as inorganic protective layers are those which are formed of dielectrics such as oxides, nitrides and sulfides (e.g., SiO.sub.x, SiN.sub.x, AlN.sub.x and ZnS). Being an oxide, SiO.sub.x has had the potential to deteriorate, rather than improve, the magnetic characteristics of the recording layer even if a protective layer made of SiO.sub.x is used. SiN.sub.x, AlN.sub.x and ZnS are non-oxides and hence provide a good barrier against air atmosphere. However, a long time has been necessary to form a layer thick enough to exhibit the desired effect. As a further problem, layers of increased thickness are prone to cracking due to internal stress and increased absorption of heat has led to a decrease in the sensitivity of magneto-optical recording media.
Known organic protective layer include those which are made of hot-melt resins, UV (ultraviolet ray) curable resins or EB (electron beam) curable resins. An example of the protective layers made of hot-melt resins is disclosed in JP-A-61-68750 (the term "JP-A" as used hereunder means an "unexamined published Japanese patent application"). These protective layers are formed on the recording layer by roll coating or some other suitable method. Although they have several advantages, such as comparatively high speed of film formation and ease of forming thick films, they have suffered from the problem that the recording layer is sometimes damaged by contact with metal rollers. UV curable resins also have several advantages that make them suitable for use in protective layers: lower initial investment is needed as compared with EB curable resins; thick films can be easily formed; the film has high heat and abrasion resistance; and pinholes are less likely to occur. Exemplary UV curable resins are disclosed in JP-A-60-117430, JP-A-61-133067, JP-A-61-144744, JP-A-62-189651, etc. Protective layers made of UV curable resins can be deposited in large thickness with comparative ease and exhibit high heat and abrasion resistance. However, protective layers made of UV curable resins have occasionally caused pinholes or corrosion in the recording layer on account of such factors as the presence of residual monomers, photoinitiators, catalysts, etc.
With a view to preventing time-dependent deterioration from occurring in phase-change recording media, protective layers have been provided by the same method as those employed with magneto-optical recording media but they have suffered from similar problems to those described above.